1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, using a crystalline thin film and, more particularly, to a method of fabricating a planar thin-film transistor. Furthermore, the invention relates to a method of fabricating a liquid crystal display making use of such semiconductor devices.
2. Description of the Related Art
In recent years, technologies for fabricating field-effect thin-film transistors (TFTs) having excellent switching characteristics on substrates having poor thermal resistance, such as glass substrates and plastic substrates, have evolved, because with the development of techniques, amorphous silicon (a-Si) thin films and polysilicon (p-Si) thin films have been formed at lower temperatures.
At present, active matrix liquid crystal displays using a-Si thin films have become dominant in flat-display technology and almost form a vast field of the electronic industry.
In an active matrix liquid crystal display, millions of pixels are arranged in rows and columns, and TFTs are disposed at each pixel. Electric charge going into and out of each pixel electrode is controlled by the switching action of the TFTs.
TFTs using p-Si thin films have high field mobilities and operate at high speeds and so they permit fabrication of an integrated liquid crystal display incorporating peripheral driver circuits.
Accordingly, a liquid crystal display using p-Si thin film is recognized as a technique for accomplishing a next-generation, high performance intelligent display. It is considered that this technique will permit fabrication of an electronic system on glass (system-on-glass).
However, silicon films have their inherent problems. Amorphous silicon thin films and low-temperature p-Si thin films have high defect level densities due to dangling bonds and crystal grain boundaries. Therefore, when TFTs are manufactured, a hydrogenation step is necessary to conduct termination by hydrogen in the active layer.
Today, hydrogenation enjoys wide acceptance because it is effective in improving the electrical characteristics of TFTs such as mobilities, threshold voltages, off currents, and subthreshold swing factors. The hydrogenation is classified into two major methods: a method using thermal processing and a method using plasma processing.
In the former method using thermal processing, a substrate to be processed is heated in a hydrogen ambient at a temperature of 300-450.degree. C. for tens of minutes to several hours to thermally diffuse hydrogen into the thin film.
In this method, in order to shorten the hydrogenation time and to lower the equipment cost, the thermal processing is preferably performed at atmospheric pressure in a 100% H.sub.2 ambient. However, since hydrogen is very active (where certain content and environment temperature are exceeded, it explodes), the Industrial Safety Standard restricts the hydrogen content severely to 3-4% or less.
Accordingly, a method consisting of performing hydrogenation in an ambient of hydrogen diluted with an inert gas and a method consisting of carrying out hydrogenation at a low pressure of hundreds of torr have been proposed. Nevertheless, both methods suffer from low hydrogenation efficiency and offer only limited industrial practicability.
Another problem is that hydrogen diffuses itself into the active layer while kept in a molecular state and thus the probability that defect levels are terminated is not very high.
The latter method relies on plasma processing. Reactant gases such as H.sub.2, H.sub.2 +O.sub.2, and NH.sub.3 are decomposed by a plasma discharge. The resulting hydrogen atoms are injected into the thin film.
In this case, the efficiency of hydrogenation is high but plasma damage and electrostatic discharge damage are induced. In addition, it is difficult to obtain optimum hydrogenation conditions.